1. Field of the Invention
The present invention relates to charging circuits charging predetermined first and second batteries and a battery charger incorporating such a charging circuit.
2. Description of the Related Art
Recently, mobile communication using mobile phones has been rapidly expanding. The reason is the convenience that users can enjoy mobile communication using mobile phones irrespective of the time and place. In order to ensure such convenience, it is required that a battery charger charging a battery used for a mobile phone can charge a spare battery as well as the battery in the mobile phone body.
FIG. 1 shows a conventional charging system 100 relating to a battery charger that charges a spare battery as well as a battery in a mobile phone body. The charging system 100 includes a spare battery 101 having a low priority in charging, a portable set 102 such as a mobile phone, a main battery 103 having a high priority in the charging provided in the portable set 102, a transistor 104, a charging circuit 105 in the portable set 102, a current detector 106, a microcomputer and control circuit 107 (hereinafter referred to as a xe2x80x9ccontrol circuit 107xe2x80x9d), a charge control circuit 108 that controls switching of the transistor 104, a transistor 109, a charge control circuit 110 that controls switching of the transistor 109, and a resistor 111 as a thermistor element for controlling the temperature of the spare battery 101.
The charging system 100 charges both spare battery 101 and main battery 103. In addition, it is assumed that, in an initial state, the transistor 104 is in an ON-state and the transistor 109 is in an OFF-state.
An AC adapter 150 converts an alternating current supplied from an alternator (not shown) to a direct current and supplies the direct current to the spare battery 101 and the main battery 103 as a charging current. The current detector 106 detects the charging current supplied to the main battery 103 and sends the detection result to the control circuit 107.
In a case where the current detector 106 is detecting the charging current, in other words, where the charging circuit 105 is charging the main battery 103, the control circuit 107 does not instruct the charge control circuits 108 and 110 to perform the switching of the transistors 104 and 109, respectively. For this reason, the ON-state of the transistor 104 and the OFF-state of the transistor 109 are maintained. Thus, charging of only the main battery 103 is continued.
On the other hand, in a case where the current detector 106 detects no charging current, in other words, where the main battery 103 is fully charged, the control circuit 107 instructs the charge control circuits 108 and 110 to control the transistors 104 and 109, respectively. According to the instruction, the charge control circuit 108 decreases the base voltage of the transistor 104. Additionally, according to the instruction, the charge control circuit 110 increases the base voltage of the transistor 109. For this reason, the main battery 103 is supplied with only a small charging current. That is, so-called trickle charging is performed. On the other hand, the charging of the spare battery 101 is started such that a large charging current is supplied to the spare battery 101.
It should be noted that in a case where the main battery 103 is removed from the charging system 100, the current detector 106 does not detect a charging current. In this case, the control circuit 107 instructs the charge control circuits 108 and 110 to control the transistors 104 and 109, respectively. That is, according to the instruction, the charge control circuit 108 controls the transistor 104 to assume the OFF-state. On the other hand, the charge control circuit 110 controls the transistor 109 to assume the ON-state according to the instruction. Thus, the charging of the spare battery 101 is started such that a large charging current is supplied only to the spare battery 101.
FIG. 2 is a graph showing the variation of the charging current with time. Here, the charging current is supplied to the spare battery 101 and the main battery 103. First, as shown in FIG. 2, the charging current is supplied to the main battery 103. Then, as the main battery 103 is fully charged, the charging current supplied to the main battery 103 is decreased, and the charging current is almost zero when the charging is completed. Thereafter, trickle charging is performed on the main battery 103. Then, the charging current is supplied to the spare battery 101.
As an example of the prior art relating to the battery charger that charges not only the battery (main battery) in the mobile phone body but also the spare battery in the above-described manner, there is a xe2x80x9cmultiple battery, multiple rate battery chargerxe2x80x9d disclosed in Japanese Patent Publication No. 8-8747 (corresponding to U.S. Pat. No. 5028859).
However, in the above-described conventional charging procedure, though the charging current that is supplied to the main battery 103 from the AC adapter 150 is decreased, the spare battery 101 is not charged at that time. Thus, effective charging is not performed. Additionally, there is also a problem in that the manufacturing cost increases by using the control circuit 107.
Accordingly, it is a general object of the present invention to provide an improved and useful charging circuit and battery charger in which the above-mentioned problems are eliminated.
It is another and more specific object of the present invention to provide a charging circuit and a battery charger that can charge batteries effectively at low cost.
In order to achieve the above-described objects, according to one aspect of the present invention, there is provided a charging circuit charging predetermined first and second batteries including: a circuit configuration by which a second charging current charging the second battery is obtained by subtracting a first charging current charging the first battery from a current supplied by a power source; and a current setting part that sets the second charging current smaller than the first charging current at least at the beginning of charging.
Also, according to another aspect of the present invention, there is provided a battery charger incorporating the above-described charging circuit.
According to the above-mentioned aspects of the present invention, the second charging current is obtained by subtracting the first charging current used for charging the first battery from the current supplied by the power source. In addition, the amount of the second charging current is set smaller than that of the first charging current at least in the beginning of charging so that the first battery is charged earlier than the second battery. Hence, it is possible to perform effective charging while continuously varying the first and second charging currents and effectively using the charging current supplied by the power source.